Table of Contents
What is a Sulphate Attack?
Sulphate attack is a significant concern for concrete floors, particularly in properties constructed between the 1940s and 1970s. During this period, industrial waste materials such as red ash (shale), black ash, slag, and grey fly ash were commonly used as hardcore infills beneath concrete slabs. These materials, often sourced from coal mines, steelworks, foundries, and power stations, were readily available and cost-effective for builders. However, they contained sulphates that, when combined with moisture, could migrate into the concrete, leading to chemical reactions that cause cracking, upward movement of the floor slab, and, in severe cases, damage to the brickwork below the damp-proof course (DPC).
The Mechanism of Sulphate Attack
When sulphate ions penetrate the concrete, they react with compounds in the cement paste, forming expansive products like ettringite. This expansion generates internal stresses, resulting in cracking and deterioration of the concrete. The presence of moisture accelerates this process, making damp conditions particularly detrimental.
How to Identify Susceptible Properties
Properties built between the 1940s and early 1970s are at higher risk, especially those without an adequate polythene membrane separating the concrete slab from the sulphate-bearing hardcore. The Building Research Establishment (BRE) began highlighting this issue in the mid-1950s, leading to changes in Building Regulations by 1967 to restrict the use of such materials and mandate protective membranes.
Investigating Sulphate Attack in Concrete Floor Slabs
Conducting a sulphate floor test is crucial for assessing the integrity of concrete floors in older properties. The sulphate level in the hardcore infill is determined by sampling it from under a floor. We also check for cracking or movement below the DPC level, whether the hardcore is dry or damp, and if there’s a polythene membrane known as a Damp Proof Membrane DPM between the concrete floor slab and the hardcore infill. The sample is sent to an independent laboratory for analysis. The result, along with site information, is incorporated into our report.
What is Sulphate Floor Testing?
The testing process involves:
Sampling: Extracting a sample of the hardcore infill from beneath the concrete floor.
Laboratory Analysis: Sending the sample to an independent laboratory for analysis using a 2:1 water/soil extract test to determine sulphate levels.
On-Site Assessment: Evaluating the presence of any cracking or movement in floors or walls below the DPC, checking for moisture in the hardcore, and verifying the existence and condition of any polythene membrane.
Measurements: Aid providing the client with a budget figure for floor replacement should a sulphate attack be confirmed.
What is contained in the Report?
The results are compiled into a comprehensive report, providing essential information for surveyors, mortgage lenders, and potential property buyers or homeowners. Our report will contain commentary on any structural implications found and confirm or disprove whether the heave and cracking were caused by sulphates. We will also provide additional commentary relating to any supplementary factors, such as leaking pipework or groundwater. A recommended budget cost to replace internal floor slabs and any other damage identified.
Sulphate Classification Levels
Sulphate levels are classified to assess potential risks:
Class 1: <0.10g SO₄/Litre to <0.50g SO₄/Litre (<100mg SO₄/Litre to 500mg SO₄/Litre)
Class 2: 0.50g SO₄/Litre to 1.50g SO₄/Litre (500mg SO₄/Litre to 1500mg SO₄/Litre)
Class 3: 1.60g SO₄/Litre to 3.00g SO₄/Litre (1600mg SO₄/Litre to 3000mg SO₄/Litre)
Class 4: 3.10g SO₄/Litre to 6.00g SO₄/Litre (3100mg SO₄/Litre to 6000mg SO₄/Litre)
Most properties tested fall into Classes 1, 2, or 3. It's important to note that the type of hardcore infill is less significant than the sulphate concentration present. Many properties with industrial fill may not pose a problem due to low sulphate levels.
In most properties tested the result will fall into Classes 1,2 or 3. It is not the type of hardcore infill that is a potential problem but the level of sulphate found in that material.
There are many properties therefore built with industrial fill under the floors that are not potentially a problem due to the low sulphate level in the material.
Class 1: This is generally acceptable to Surveyors and mortgage lenders in North Wales, North West and the Midlands as the level of sulphate is not considered to be high enough to cause damage to the floors and walls of the property in the future.
Class 2: Is the grading that poses the most problems for Surveyors and Mortgage Lenders. Generally, a low Class 2 result is acceptable especially if there is no evidence of damage and the hardcore is dry. The presence of a polythene DPM will provide additional protection. A high Class 2 result increases the potential for damage although if the hardcore sample is dry and there is no evidence of damage due to sulphate action then the chances of any damage occurring in the future are still very small. Any decision however concerning the removal of the floors and hardcore infill is at the discretion of the Surveyor and/or Mortgage Lender.
A Class 3 result is generally not acceptable especially if there is no polythene DPM present between the hardcore infill and the concrete floor slab. This method will apply to all properties built with traditional foundations. The property may however have been built on a raft foundation due to poor ground conditions, mining subsidence etc. Where a raft foundation is found it is usually not necessary to take and test a sample of the hardcore infill as the amount of hardcore infill under the raft is likely to be very small, the raft will be usually a minimum of 200mm thick, will be a stronger concrete mix than in a normal traditional floor and will be reinforced with steel mesh reinforcement. Taking all these factors into consideration we interpret current BRE guidance as indicating the Building Research Establishment says that even if sulphate-containing industrial fill is present under the raft foundation, the rate of attack on the concrete is unlikely to exceed 1mm per year making the risk of damage minimal.
What is a Damp Proof Membrane DPM?
When It became apparent this infill material could cause damage to the concrete floors and brickwork below the DPC level of a property due to the sulphate present in the material. The Building Research Establishment (BRE), started issuing information on this problem in the mid-1950s and in 1967 the Building Regulations changed to restrict the use of industrial materials as hardcore infill and a 500g polythene membrane was introduced to provide a barrier between the concrete floor slab and the hardcore infill. DPMs then increased in thickness to l000g in 1970 and 1200g in the early 1980s.
The BRE regarded the 500g membrane as an acceptable membrane barrier between the existing concrete floor slab and the hardcore infill. The minimum requirement to comply with current Building Regulations for new build or replacement floor construction is 1200g polyethene sheet DPM complaint with NHBC Technical Requirement R3 or bitumen sheet to BS 6398. Although the use of industrial waste materials which had high sulphate levels was outlawed in the 1967 Building Regulations, its use can still be found in properties built up to circa 1970. Surveyors may therefore request tests for properties built up to circa 1972.
Conclusion
In conclusion, sulphate attack poses a significant threat to the structural integrity of concrete floors, especially in properties constructed between the 1940s and 1970s using industrial waste materials as hardcore infill. The chemical reactions between sulphates and concrete components can lead to cracking, heaving, and severe damage if left unaddressed.
Regular sulphate floor testing is essential for early detection and prevention of potential issues. By identifying sulphate concentrations and assessing the condition of damp-proof membranes, property owners can implement appropriate remedial measures to safeguard their investments.
At AMS Surveys, we specialise in comprehensive sulphate floor testing and reporting. Our team of Chartered Building Surveyors is equipped with the expertise to accurately diagnose and provide solutions for sulphate-related problems, ensuring the longevity and safety of your property.
Don't wait for visible signs of damage. Proactive testing and maintenance are key to preventing costly repairs and preserving the value of your property.
How AMS Surveys Can Help with Investigation
If you own a property built between the 1940s and 1970s, especially in areas with a history of industrial activity, it's prudent to have your concrete floors tested for sulphate content.
Contact AMS Surveys today to schedule a comprehensive sulphate floor test and ensure the longevity and safety of your property.
Frequently Asked Questions (FAQs) on Sulphate Attack and Testing
1. What is sulphate attack in concrete floors?
Sulphate attack refers to the deterioration of concrete floors caused by chemical reactions between sulphate ions and compounds in the cement paste. This reaction leads to the formation of expansive products like ettringite, resulting in cracking, swelling, and structural damage.
2. Which properties are at risk of sulphate attack?
Properties constructed between the 1940s and early 1970s are particularly susceptible, especially those that used industrial waste materials such as red ash, black ash, slag, and grey fly ash as hardcore infill beneath concrete slabs. The absence of an adequate damp-proof membrane (DPM) further increases the risk.
3. How can I identify if my property has a sulphate attack issue?
Signs include:
Cracking or upward movement of concrete floors.
Damage to brickwork below the damp-proof course (DPC).
Presence of moisture in the hardcore infill.
Lack of a polythene membrane between the concrete slab and hardcore.
A professional assessment is recommended for accurate diagnosis.
4. What does sulphate floor testing involve?
The process includes:
Sampling: Extracting a sample of the hardcore infill from beneath the concrete floor.
Laboratory Analysis: Testing the sample to determine sulphate levels using a 2:1 water/soil extract test.
On-Site Assessment: Evaluating structural integrity, moisture presence, and the existence of a damp-proof membrane.
Reporting: Compiling findings into a comprehensive report with recommendations.
5. What are sulphate classification levels, and what do they indicate?
Sulphate levels are categorized to assess potential risks:
Class 1: <0.10g SO₄/Litre to <0.50g SO₄/Litre (<100mg SO₄/Litre to 500mg SO₄/Litre)
Class 2: 0.50g SO₄/Litre to 1.50g SO₄/Litre (500mg SO₄/Litre to 1500mg SO₄/Litre)
Class 3: 1.60g SO₄/Litre to 3.00g SO₄/Litre (1600mg SO₄/Litre to 3000mg SO₄/Litre)
Class 4: 3.10g SO₄/Litre to 6.00g SO₄/Litre (3100mg SO₄/Litre to 6000mg SO₄/Litre)
Higher classes indicate greater sulphate concentrations and increased risk of damage.
6. What is a Damp Proof Membrane (DPM), and why is it important?
A DPM is a polythene sheet placed between the concrete floor slab and the hardcore infill to prevent moisture and sulphates from migrating into the concrete. Its use became standard in the late 1960s to mitigate sulphate attack risks.
7. How can AMS Surveys assist with sulphate floor testing?
AMS Surveys offers comprehensive sulphate floor testing services, including sampling, laboratory analysis, on-site assessments, and detailed reporting. Our expertise ensures accurate identification and effective mitigation of sulphate-related issues on your property.
For more information or to schedule a survey, please contact us at 0151 314 6650 or visit www.amssurveys.co.uk
Comments